Synthetic aperture analysis radar

ABSTRACT

A SYNTHETIC APERTURE ANALYSIS RADAR SYSTEM IN WHICH THE TRANSMITTED AND RECEIVED SIGNALS ARE NOISE SIGNALS. THE RETURN SIGNAL IS ADDED TO A REFERENCE SIGNAL DERIVED FORM THE TRANSMITTED SIGNAL. UNDER CERTAIN CONDITIONS THE POWER SPECTRUM OF THE SUM SIGNAL IS MODULATED. SYNTHETIC APERTURE PROCESSING CAN THEN BE PERFORMED SINCE THE POWER SPECTRUM IS ESSENTIALLY COHERENT.

United States Patent Appl. No. Filed Patented Assignee SYNTHETICAPERTURE ANALYSIS RADAR [56] References Cited UNITED STATES PATENTS3,121,868 2/1964 Hausz etal. 343/5 Primary Examiner-T. H. TubbesingA!t0rneysHarry A. Herbert, Jr. and George Fine ABSTRACT: A syntheticaperture analysis radar system in 3 chimss Dn'hg Figs which thetransmitted and received signals are noise signals. US. Cl 343/51}, Thereturn signal is added to a reference signal derived from 343/175 thetransmitted signal. Under certain conditions the power Int. Cl G015 9/02spectrum of the sum signal is modulated. Synthetic aperture Field ofSearch 343/5, 5 processing can then be performed since the powerspectrum is (CM), 17.5 essentially coherent.

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SHEET 2 BF 2 X YNkWNQ VIII II '11! 1| Ill] Ill: ||||11 :llilllllIlliLSYNTHETIC APERTURE ANALYSIS RADAR BACKGROUND OF THE INVENTION Thisinvention relates to radars and more particularly synthetic apertureanalysis radars in which a radar beam is formed synthetically. Thesynthetic aperture technique is basically an arraying technique. In anordinary radar array antenna many elements are spaced along the apertureand the energy from each element is phased properly in order to addvectorially to the energy from the other elements in such a manner as toform a beam. In a synthetic array radar only one element is used and itis moved from place to place. The signal from each element is thenstored until the element is moved through all the positions. The signalsare then shifted in phase so as to add up properly to form the beam. Thevehicle to move the element is usually an aircraft. although any methodofimparting motion may be used.

The present invention contemplates the use of noise signals in syntheticaperture radars. This has many important operational advantages, such aslow mutual interference and reduced fluctuations. Fluctuations in thereturn signal is the principal shortcoming of conventional radars andlimits the maximum resolution obtainable. The inventor solves theproblem of synthetic aperture processing of noise or noncoherent radarsignals. This has been heretofore impossible except in the case of apseudorandom radar system.

SUMMARY OF THE INVENTION The invention is a radar system in which thetransmitted and received signals are noise signals. The return signal isadded to a reference signal derived from the original transmittedsignal. Under certain conditions the power spectrum of the sum signal ismodulated. However, unlike other noise radars, synthetic apertureprocessing can be performed since the power spectrum is essentiallycoherent. Since the power spectrum is a coherent waveform it isprocessed to form a focused synthetic aperture beam.

An object of the invention is to provide a synthetic aperture spectrumanalysis radar to form a focused synthetic aperture beam.

Yet another object of the present invention is to provide a radar systemwith the power spectrum thereof being a coherent waveform that isprocessed to form a focused synthetic aperture beam.

Yet another object of the present invention is to provide a radar systemfor the synthetic aperture processing of noise and noncoherent radarsignals.

DESCRIPTION OF THE DRAWINGS FIG. 1 is a block diagram of the basicspectrum analysis radar;

FIG. 2 illustrates the geometry of the problem of synthetic apertureprocessing; and

FIG. 3 is a block diagram of synthetic processor for a spectrum analysisradar.

DESCRIPTION OF THE PREFERRED EMBODIMENT FIG. 1 is a block diagram of thebasic spectrum analysis radar before synthetic aperture processing.Noise source generates a band limited noise signal which is transmittedby antenna 13 toward target 14 after passing through directional coupler11 and circulator 12. A small preselected portion of the transmittedsignal is coupled off by directional coupler 11 and is added to thereturn signal from antenna 13. This is accomplished by the return signalpassing from antenna 13 to circulator 12 through isolator 15 to summer17 while a portion of the band limited signal from noise source 10 isobtained from directional coupler 11 which then passes through isolator16 to summer 17. The power spectrum of this summed signal is given bywhere R is the range to the target, c is the speed of light, A is themagnitude of the power spectrum, and for a constant reference isproportional to the return from the target, f is frequency, and i is thephase shift due to the target. The summed signal is then received byspectrum analyzer 18to provide target information. Spectrum analyzer 18also provides a synchronization output at terminal 19.

The use of this system with synthetic aperture processing can best beunderstood by analyzing the geometry of the problem by referring to FIG.2. A vehicle, usually an aircraft, carrying the radar system, flies atvelocity v from point P =O to P,=vt. One wishes to focus a syntheticbeam at point x y The range to x,, y, is RQ) and is given by RUFNW (2)This can be written as With the binomial expansion and neglecting higherorder terms this expands to The preceding can be shown to be equal toR(t)=R +41i-vt sinag (5) If We substitute (5) into (1) we see that asthe vehicle moves from P to P; we have 21 1 cos 0 Thus we see that thepower spectrum is not only a function of the frequency f, but as theaircraft flies along, the power spectrum becomes a function of time andthe angle 9 from the aircraft to x y as Well as the range R,,. Thus thisfunction can be used to obtain angular resolution, A. This can beaccomplished in the following way.

FIG. 3 is a block diagram of a synthetic aperture processor for spectrumanalysis radar 20 which was shown in block diagram form in FIG. 1.Reference function generator 21 has as its output,

18 and d are predetermined and are the range and azimuth angles to thepoint it is desired for the radar system to look at. S; andiS' are thenmultiplied by S in multipliers 22 and 23, the output of spectrumanalysis radar 20. Signals returning from the point x y, will cause a DCvoltage after multiplication while those at other ranges and/or angleswill have no DC component but only a sinusoidal component and so will berejected. The outputs of multipliers 22 and 23 are passed throughlow-pass filters 24 and 25 to squarers 26 and 27, respectively. Theoutputs of squarers 26 and 27 are added in summer 28 and then passedthrough square root circuit 29. Thus, in order to determine A, we squareA cos 1 and A sin 1%, sum the squares and take the square root. Theoutput of square root circuit 29 is shown on conventional display 30.Note that timing from the spectrum analyzer synchronizes the referencefunction generator. The above setup will give the magnitude of thereturn from targets located at only 5 and R In order to be more useful,returns from many different angles and ranges should be processed. Thiscan be done quite simply by having many different reference functionsand many channels of multiplication. An excessive amount of hardware isnot necessary if the functions enclosed by the dotted lines areperformed'by a special purpose digital computer. It would then bepossible to process the different functions sequentially.

It is noted that the reference function generator is conventional andmay be an oscillator providing a predetermined linear FM signal,

We claim:

I. A synthetic aperture spectrum analysis radar comprising means togenerate a noise signal, means to transmit said noise signal toward atarget of interest at a preselected position to obtain a return target,signal and to receive the return target signal, first means to sum saidreturn target signal with a preselected portion of the transmitted noisesignal to obtain a modulated power spectrum, means to analyze thespectrum of the sum signal to obtain a resultant signal, a functiongenerator providing a predetermined signal, first and second means tomultiply said resultant signal with said predetermined signal, first andsecond means to square the outputs of said first and second multipliers,respectively, second means to sum the outputs of said first and secondsquaring means, and means to obtain the square root of the output signalof said second summing means 2. A synthetic aperture spectrum analysisradar as defined in claim 1 further including means to display theoutput signal from the square root means.

3. A synthetic aperture analysis radar as defined in claim 2 furtherincluding a first low-pass filter interposed between said firstmultiplying means and said second summing means, and a second low-passfilter interposed between said second multiplying means and said secondsumming means.

